High-resolution still images are desirable because of the greater detail that is captured when compared to low-resolution still images. Fortunately, high-resolution cameras that are able to capture these high-resolution still images are becoming more accessible to the average consumer. With the advent of “prosumer” digital cameras, a user can buy a high-resolution camera containing several features once available only on more expensive professional cameras.
Unfortunately, however, many types of undesirable artifacts become more pronounced with high-resolution images. One of these artifacts is a phenomenon known as “purple fringing”. Purple fringing exhibits itself as an out of focus purple ghost image rimming parts of the image. Although there is debate about the exact causes, it is known that the primary cause of purple fringing is chromatic aberration. Chromatic aberration is light dispersion by the lens. It occurs when different wavelengths of light focus at different spatial locations in the image, which results in a shift in colors. Purple fringing is more pronounced at sharp edges at object boundaries. Other secondary causes of purple fringing include blooming, demosaicing (for single CCD cameras), and anti-aliasing filters.
Several techniques are available that claim to eliminate purple fringing in high-resolution images. However, the majority of these techniques require manual intervention and input from a user. For example, typical purple fringing techniques require the user to manually indicate the purple-fringing region in the image. Moreover, these techniques require manual specification of the color that the user wants to replace the purple-fringed region. Thus, there is a dearth of automatic purple-fringing removal techniques.
One recent technique that comes closer to automatic purple-fringing removal is a plug-in for a popular image processing software application. However, one drawback with this current technique is that the purple-fringed regions in an image are not automatically detected. The user must indicate to the plug-in the location of purple-fringed regions before the regions are automatically corrected. Another disadvantage is that this technique merely replaces the purple-fringed regions by another color to tone down the purple-fringing effect. This causes problems if the purple-fringed region is near an object in the image having a true color that is near purple, because the original color is altered considerably. Yet another shortcoming is that there is no portion of the purple-fringed region that is replaced with an entirely monochromatic (or black-and-white) region. Thus, purple-fringing is not totally eliminated.